5.7 Given a scenario, troubleshoot problems with wired and wireless networks

Common symptoms
- Limited Wireless Connectivity
- Slow Network Speeds
- Limited Connectivity
- Jitter
- Poor voice over Internet Protocol (VoIP) Quality
- Port Flapping
- High Latency
- External Interference

Troubleshooting Wi-Fi/Network

As discussed earlier, there are seven layers to a network. When troubleshooting a network issue, you should begin troubleshooting at Layer 1 and work your way up to Layer 7, until you have resolved the issue.

Layer 1 Physical Layer	This layer deals with physical connections. Are the devices physically connected to each other? Test the ethernet to make sure that the continuity is good on all pairs. Check that all the devices are powered on correctly. Check that the network interface cards are functioning.
Layer 2 Data Link Layer	This layer deals with Ethernet and Point to Point Protocols between two devices A cable with good continuity may still have interference from electrical or other sources. Check that the interfaces are connected at the correct speed (10, 100, 1000, etc.) and the correct duplex (full duplex or half duplex). Both devices should have the same speed and duplex settings. Remember that a network interface’s speed and duplex could be set manually or automatically. Check that the device is connected in the correct VLAN.
Layer 3 Network Layer	This layer contains the IPv4 and IPv6 Protocols Check that all the devices have the correct IP addresses, gateway addresses, and DNS servers Check other routing protocols to make sure that they are connected correctly Check that the device has received an address over DHCP Check that nothing is being blocked by a firewall
Layer 4 Transport Layer	This layer delivers data from the network card to the application Check that the applications are configured correctly, and with the correct port number Check that nothing is being blocked by firewalls or antivirus programs
Layer 5 Session Layer	The session layer involves authentication. Check that the applications are configured correctly and that the correct credentials (URLs, usernames, passwords, certificates, etc.) have been entered into the applications
Layer 6 Presentation Layer	Sometimes the presentation layer and application layers are combined The presentation layer is responsible for formatting received information and sending it to the application layer Check that the application is configured correctly.
Layer 7 Application Layer	The application layer is the user interface that displays received information to the user Check that the application is configured correctly

What are some common network issues?

Limited Connectivity	The device is not able to reach the internet. If the device is set to obtain an IP address via DHCP, this issue is typically associated with an APIPA/link local address. If the device is not able to reach a DHCP server, check that the DHCP server is configured, online, and not being blocked. If the device is configured to use a static IP, ensure that the static IP address is in the correct subnet and has the correct gateway. Check that the DNS servers are correct and reachable. Check that no firewall is blocking the connection. Check that the network is connected to the internet. Confirm that the modem or WAN connection is operating correctly.
Unavailable Resources	The resources (such as a printer or file server) may be powered off or disconnected from the network. Check that the resources are available and that their network connection is configured correctly. Check if the other resources are functioning correctly, and that no security appliances are blocking the connection. Verify that no IP address conflict exists – that is, verify that two devices are not configured to use the same IP address.
No Connectivity	The device is not connected to the network. Check that the cable is connected, the network switch is powered on, and that the network interface is connected. Verify that the ethernet cable is not damaged.
APIPA/Link Local Address	The device is not able to reach a DHCP server. Check that the DHCP server is functioning and connected to the network.
Intermittent Connectivity	The network switch is failing, or the cable connection is damaged. Replace the cable or network switch. Check that the Wi-Fi signal is adequate and not being blocked by obstructions.
IP Conflict	Multiple devices on the same network have the same IP address. This typically occurs when the addresses are statically assigned (since a DHCP server will automatically check for conflicts) Change the devices to DHCP (if available) or change one of the static IPs.
Slow Network Speeds	Find out where the bottleneck is in the network topology. Is it the computer, the local network, or the internet? If only one resource (server, website, etc.) is slow, then the issue is with the resource and/or the route that traffic is taking to the resource. If other many resources are slow, the internet connection might be slow. Check the connection directly from the internet modem. The router may be overloaded. Reboot the router or replace it with a more powerful one. Verify that the computer is not slow. Check if the computer is providing slow speeds, or programs running in the background are affecting it.
Low RF Signal	The Wi-Fi signal is poor. The Wi-Fi signal can be reduced by walls, concrete, and steel. The Wi-Fi access point may be too far away or may not have adequate antennas. Move to an area with better signals or install additional access points.
SSID Not Found	The SSID is not correct, or the wireless access point is not broadcasting the SSID. Check the configuration on the Access Points.
Jitter	Jitter happens when packets do not arrive in the same order that they were sent. We can install a buffer to resolve the jitter issues. The buffer traps packets when they arrive and reorders them. We could also purchase a more reliable internet connection.
High Latency	Latency happens due to a delay in receiving data packets. We can improve latency by purchasing a more reliable internet connection.
Poor VoIP Quality	Poor VoIP call quality comes from high jitter, high latency, and/or low bandwidth. VoIP traffic is more sensitive to jitter and latency than other applications because its data must be played back in real time. We can improve the VoIP call quality by purchasing a more reliable internet connection. We can also set our router to prioritize VoIP traffic.
Port Flapping	Verify that the cables and jacks between the two devices are not damaged. Verify that the speed and duplex settings are the same on both devices.
External Interference	External interference can be corrected by moving the network equipment away from any power source.